1. Field of the Invention
This invention relates in general to semiconductors and semiconductor devices and more particularly, to methods for growing nitrogen-containing alloy semiconductors and semiconductor devices such as semiconductor lasers and light emitting diodes comprising the alloy semiconductors.
2. Description of the Related Art
Compound semiconductors in general and nitrogen-containing III-V alloy semiconductors in particular have recently received considerable attention as a group of new semiconductor materials.
As an example of methods for growing a III-V alloy semiconductor, there has been disclosed an epitaxial growth of a nitrogen-containing III-V alloy semiconductor on a silicon substrate (Japanese Laid-Open Patent Application H6-334168). In the disclosure, a nitrogen-containing alloy semiconductor is epitaxially grown without dislocations by the lattice misfit, thereby leading to feasibility of fabricating III-V compound semiconductor devices on silicon.
There are also disclosed nitrogen-containing alloy semiconductors such as GaInNAs, AlGaNAs, and GaNAs, which can be lattice matched to substrates such as GaAs, InP, and GaP (Japanese Laid-Open Patent Application H6-037355). Although it has been previously considered that no known III-V alloy semiconductor is grown, which had a smaller bandgap energy than that of GaAs and lattice matched to GaAs, recent results of the alloy semiconductor growths indicate that the above-mentioned GaInNAs alloy semiconductor, for example, can be grown as a material lattice matched to the GaAs substrate.
Since the GaInNAs compound semiconductor has a smaller bandgap energy than that of GaAs, above growth results of GaInNAs alloy semiconductor may lead to the feasibility of light emitting devices, of which wavelength of the light emission is in the range of about 1.5 micron, longer than that from GaAs. As aforementioned, that has been previously conceived not feasible by the devices fabricated on GaAs substrates.
In the growth of the above-mentioned nitrogen-containing III-V alloy semiconductors, a group V element such as arsenic, for example, is desorbed from the surface of a substrate with ease even at relatively low temperatures. Therefore, the alloy semiconductors are preferably grown at temperatures as low as possible.
As a source material for nitrogen, NH.sub.3 has been used conventionally. However, the use of NH.sub.3 is not completely satisfactory, since temperatures for alloy semiconductor growth have to be relatively high because of a relatively high dissociation temperature of NH.sub.3 and an unduly desorption of arsenic results at such temperatures. Therefore, NH.sub.3 is not preferred as a source material for the growth of alloy semiconductors which simultaneously contain both nitrogen and arsenic.
Accordingly, in place of the use of NH.sub.3 as is in conventional growths, active species which are generated by a high frequency plasma excitation of nitrogen, have been used as the source material for nitrogen. These species are used for a growth of nitrogen-containing III-V alloy semiconductors by, for example, molecular beam epitaxy (MBE) which is carried out under very low pressures, or low pressure metalorganic chemical vapor deposition (MOCVD) which is performed at pressures of about 0.1 Torr(13.3 Pa).
As another source material for nitrogen, an organic material such as dimethylhydrazine (DMHy) has been reported for a growth of a GaNAs alloy semiconductor by conventional MOCVD at a reduced pressure of 60 Torr (as described by N. Ohkouchi and others, Proceedings of 12th Symposium on Alloy Semiconductor Physics and Electronics, 1993, pages 337-340)
As aforementioned, nitrogen is desorbed from crystal surfaces with relative ease during the growth. This causes difficulties in obtaining nitrogen-containing III-V alloy semiconductors with a large concentration of nitrogen. In conventional growths of the alloy semiconductors, therefore, previous attention is primarily directed to an increase of the nitrogen content during the crystal growth.
In the above-mentioned method using activated nitrogen species, a partial pressure of nitrogen during the growth had to be relatively low to avoid deactivation of the nitrogen species previously activated. Therefore, for the growth of nitrogen-containing alloy semiconductor such as GaNAs, for example, the low partial pressure of nitrogen necessarily leads to low pressures of another group V element such as arsenic. This results in an undesirable increase in concentrations of arsenic vacancies and makes it difficult to grow nitrogen-containing III-V alloy semiconductor of satisfying quality.
For example, in order to obtain the nitrogen content of about 1%, a crystal growth is carried out under the conditions such as a reactor pressure of 25 Pa, N.sub.2 flow rate of 50 sccm, and AsH.sub.3 flow rate of 10 sccm (as described by M. Sato, Proceedings of 13th Symposium on Alloy Semiconductor Physics and Electronics, 1994, pages 101-101). Although the reactor pressure is once increased to 300 Pa to initiate the plasma discharge in the reactor, a plasma activation of nitrogen is subsequently carried out at 25 Pa, as indicated above. Under the conditions, a partial pressure of AsH.sub.3 of as low as approximately 0.9 Pa results. In addition, other gaseous materials such as triethylgallium (TEG) as a source material for group III element and H.sub.2 as a carrier gas, have to be additionally supplied into the reactor. As a result, the partial pressure of AsH.sub.3 further decreases.
In order to further increase the nitrogen content, it is necessary either to reduce the reactor pressure, increasing the amount of nitrogen source materials, or decreasing the flow rate of AsH.sub.3. The partial pressure of AsH.sub.3 further decreases.
As exemplified as above, previous growth attempts to increase the nitrogen content had led to an increase in the concentration of arsenic(group V element) vacancies, and this gives rise to, in turn, metal (group III element)-rich alloy semiconductors. As a result, these methods are not able to provide nitrogen-containing alloy semiconductors of excellent quality.
As another example of the nitrogen source material, the aforementioned dimethylhydrazine (DMHy) is also used for a growth of a GaNAs alloy semiconductor by conventional low pressure MOCVD operated at a pressure of 60 Torr, resulting in the nitrogen content of about 0.5% or less. This GaNAs growth has been carried out also under conditions of an increased flow rate of DMHy and a decreased flow rate of AsH.sub.3. Because of a decreased AsH.sub.3 partial pressure, a metal-rich alloy semiconductor is obtained similarly to the above example and even under the condition of an increased nitrogen flow rate, it is difficult to increase the nitrogen content and this method also is not be able to provide nitrogen-containing alloy semiconductors of excellent quality.